In the manufacture of semiconductor devices, crystals of semiconductor material are grown to produce a cylinder that is sliced perpendicularly to its axis to form wafers. Doped layers are formed on a wafer as required for the type of device involved, and the wafer with its doped layers is then cut into rectangular sections or chips to form identical devices or chips by a process referred to as dicing.
Dicing apparatus described in U.S. Pat. No. 3,478,752 is comprised of spaced parallel drums having helical grooves in which between a supply reel and a take-up reel is wound so as to only contact the remote sides of the drums. The wire sections between the tops and bottoms of the drums are placed under tension by forcing the drums apart. The flat wafer to be diced is adhered by wax to a flat glass slide or base that is held in position by a vacuum chuck. The sections of the wire that are tangent to the bottoms of the drums as well as the section between the top sides of the drums are in planes that are parallel to the planar surface of the wafer, and channels are cut through the wafer by forcing the open wire sections having portions tangent to the bottoms of the drums wire into contact with it. When the wire sections emerge from the other side of the wafer, they pass into the wax support. The wires are then drawn back through the channels they have just cut, the wafer is rotated by 90.degree., and the wires are once again brought to bear against the wafer to cut orthogonal channels therethrough so that the wafer is diced.
Because of the helical progression of the wires about the drums, there is a lateral force between each wire section and one sidewall of each channel it is cutting that can cause the strips into which the wafer is cut after the first cutting to become misaligned just as the cutting is finished. Thus the location of the channels formed by the second cutting may not be truly orthogonal. This is possible because the wires can exert lateral forces on the strips through the wax. In fact, the strips may be turned at different angles so that the sides of the chips formed during the second cutting may have respectively different angles with respect to the sides cut during the first cutting. Thus the semiconductor devices formed by dicing the wafer may not be identical as is desired.
Furthermore, regardless of the tension on the wires, they start cutting the wafer at its outer edges, and the cutting gradually proceeds to the center of the wafer so that the wires may have a component of bending or bowing in a plane that is vertical with respect to the plane of the wafer. Because of the helical winding of the wire about the drums, the wire turns may also have a component of bending or bowing in a plane parallel to the wafer that causes the channels to be other than vertical. Thus, in these or other ways, the unavoidable bending or bowing of the wires can result in the devices cut from the wafer being nonuniform.